Project Summary The primary risk factor for most neurodegenerative diseases and many other human ailments such as cancer is old age. A major challenge in studying late-onset diseases is the accurate representation of the aging context in both in vitro and in vivo models of the disease. This is due in part to a lack of understanding of the cellular and molecular characteristics of the aging process. We previously defined a set of cellular changes associated with aging in cells from old donors and demonstrated that these age-related hallmarks are restored to a youthful state through reprogramming into induced pluripotent stem cells (iPSC) and maintained in such ?rejuvenated? state upon re-differentiation into iPSC-derived cells. This phenomenon, while fascinating from a scientific perspective, also represents a concrete barrier for the use of iPSC for studying age-dependent disorders. Our group demonstrated that these aging characteristics can be reintroduced into iPSC-derived cells through simple expression of progerin, a mutant form of the lamin A (LMNA) protein that is responsible for the premature aging disorder Hutchinson-Gilford Progeria. However, inducing cellular age using a disease-causing factor may not be a faithful representation of physiological aging and potentially lead to pathological artifacts in modeling aging-related diseases. It is therefore necessary to develop a cellular model that will accurately reproduce the physiological aging context. Our hypothesis, supported by promising preliminary results, is that cellular aging is promoted by specific genetic and epigenetic changes that can be utilized to trigger an aged cellular state in models of late-onset disease. Here, we propose to develop a new approach for modeling age in iPSC-derived neuronal lineages in three specific steps. First, we aim to generate a comprehensive characterization of genetic and epigenetic features of aged cells using primary fibroblasts and brain tissues from young and old donors, which will provide genomic aging signatures of different cell lineages. We will then monitor those signatures during reprogramming and identify candidate determinants of cellular age. We will validate these signatures by targeted experiments as well as on independent samples. Second, using a combination of our previously identified cellular hallmarks of aging in conjunction with the newly identified genetic and epigenetic aging markers we will design optimal strategies to induce cellular aging. Third, we will use these strategies to study the impact of cellular age on the progression and pathology of Parkinson disease (PD) using iPSC-derived dopamine neurons from patients with genetic forms of PD. These will both be used for in vitro studies characterizing cellular PD manifestations as well as in vivo upon transplantation into PD mouse models to assess the impact of induced aging on cellular behavior and function in vivo. The results from this study will provide a more comprehensive understanding of the genetic and epigenetic changes that underlie human aging and importantly, provide a methodology to induce aging context in iPSC models of human diseases.